[LOCAL] arm64: renesas: defconfig: Update for renesas-drivers

Update the defconfig for Renesas ARM64 systems:
  - Compile the kernel with warnings as errors,
  - Enable kexec file based system call (kexec is cool),
  - Replace CONFIG_PCI_PWRCTRL_SLOT=y by CONFIG_PCI_PWRCTRL_GENERIC=y
    (cfr. commit ff124bbbca1d3a07 ("PCI/pwrctrl: generic: Rename
    pci-pwrctrl-slot as generic")).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>